1. Field of the Invention
The present invention relates to an optical pickup device and an optical disk drive that are mounted on an electronic apparatus, such as a personal computer or a notebook computer.
2. Description of the Related Art
Conventionally, the downsizing of electronic apparatuses, such as personal computers and notebook computers, has been expedited, and accordingly, the downsizing of optical pickup devices and optical disk drives to be mounted on these apparatuses has also been advanced.
FIG. 18 is a diagram showing the arrangement of the essential portion of the optical system of a conventional optical pickup device. A laser source 101 emits, toward an optical disk 104, a laser beam, for a DVD, having a wavelength λ1 of about 650 nm and a laser beam, for a CD, having a wavelength λ2 of about 780 nm. A prism 102 is formed of optical glass, for example, and internally includes a slope on which a beam splitter 103 is formed. The beam splitter 103 is a polarizing separation film, for which one property is that while a laser beam emitted by the laser source 101 is permitted to pass and continues on to the beam disk 104, a laser beam that is reflected by the optical disk 104 is reflected. The optical disk 104 is a DVD or a CD. A detection lens 105 is an astigmatism generation element having the shape of a so-called columnar lens or a cylindrical lens, and a focal length that differs for two intersecting cross sections that include light axes. A light receiver 106 has a photo detector 107 for receiving light reflected by the optical disk 104.
The laser beam having the wavelength λ1 and the laser beam having the wavelength λ2, both emitted by the laser source 101, pass through the beam splitter 103 and strike the optical disk 104. The laser beams reflected by the optical disk 104 are reflected by the beam splitter 103, and the reflected beams are transmitted to the detection lens 105 and then enter the light receiver 106.
FIG. 19A is an explanatory diagram for a conventional detection lens, FIG. 19B is a diagram showing the state of a spot when an optical disk is located nearby, and FIG. 19C is a diagram showing the state of a spot when an optical disk is located at a distance. In FIG. 19A, the arrangement of detection lens 105 is such that a cross section in the vertical direction is focused before beams enter the light receiver 106, and a cross section in the horizontal direction is focused after the beams enter the light receiver 106. That is, the light receiver 106 is centrally located, between the two focuses, and the shape of a spot 108 on the light receiver 106 is almost circular. When, as shown in FIG. 19B, the optical disk 104 is located near the optical pickup device, the spot 108 formed by laser beams on the light receiver 106 is extended horizontally. On the other hand, when, as shown in FIG. 19C, the optical disk 104 is located at a distance from the optical pickup device, the spot 108 formed by laser beams on the light receiver 106 is extended vertically. When the photo detectors A to D 107 are arranged in a cross shape and calculate a focus error signal FES=(A+C)−(B+D), a focus control signal can be obtained. That is, when the optical disk 104 is located nearby, the focus error signal FES>0 is obtained, and when the optical disk 104 is located at a distance, the focus error signal FES<0 is obtained, which together enable the location of the optical disk 104 to be identified.
According to (patent document 1), an optical member is additionally provided between the beam splitter 103 and the detection lens 105 in order to reduce the intensity at the center portion of the light reflected by the optical disk 104. As a result, the possibility is reduced that a track crossing signal will occur that enters a focus control signal.
Patent Document 1: JP-A-6-309687
The thickness of the detection lens, however, must be reduced in order to further provide for the downsizing of the optical pickup device and the optical disk drive. But when the detection lens is thinned, a satisfactory difference is hard to obtain between the focal lengths of the two intersecting cross sections that include the light axes, and the sensitivity of a focus control signal is reduced.